It's no secret that Djokovic has had a breakout season, or that he has been, by any reasonable standard, the world's best athlete of 2011. On Sunday, he beat Rafael Nadal in the Rome Masters, his fourth-straight win over the Spaniard. It was his second win over Nadal on clay in two weeks, and again, amazingly, he did it without losing a set. The match ran Djokovic's 2011 record to 37-0 with seven titles.

As the French Open begins Sunday, Djokovic's amazing streak—the longest to start a season since 1984—is threatening to push Roger Federer (the winner of a record 16 Grand Slam titles) and Nadal (the French Open's five-time champion) off the front pages. But the transformation from odd man out to invincible overlord also is leaving gobsmacked tennis fans searching for answers. Clearly something has clicked for the Serb. But what?

Djokovic's serve, sloppy as recently as last season, is now precise, fluid and, at times, devastating. His forehand used to break down in tense moments; now he hits winners that seem to subscribe to undiscovered laws of physics. His backhand, always solid, is now impenetrable, even with Nadal's famously high-bouncing forehand. And then there's the gluten.

Last year, Djokovic's nutritionist discovered that Djokovic is allergic to the protein, which is found in common flours. Djokovic banished it from his diet and lost a few pounds. He says he now feels much better on court.

A gluten-free diet can have implications far beyond the physical, especially in tennis, which taxes the mind like few other sports. The season is 11 months long, matches are grueling and can last for hours, and the slightest dip in a player's confidence can derail months of hard work. There's never anyone else to blame for a match gone awry.

"It's mostly mental energy you're talking about, not energy supplied to muscle tissues," said David Levitsky, a professor of nutrition and psychology at Cornell University, when asked about the effects of giving up gluten if one has an allergy. (An allergy differs from celiac disease, whose sufferers, Levitsky said, can incur far-reaching health effects from eating gluten, including the inability to absorb nutrients.)

Levitsky said a gluten-free diet might have benefits for those with mild allergies, or even no allergy at all. "The other part of the story is, if you believe in a cause of your disorder, it becomes the cause," he said. "We see this in many different studies. If you believe it, you change your behavior in the direction of being cured."

In tennis, something small often leads to a big boost in confidence. For Djokovic, there has been no shortage of pick-me-ups in the past nine months.

It's not every day that Federer loses a match point in a U.S. Open semifinal. Last year against Djokovic, he lost two. By the luck of the Davis Cup draw, Serbia played two home matches at the end of last year and eventually won its first Davis Cup title, with Djokovic leading the way. As his confidence surged and his fitness improved, Djokovic arrived in Melbourne with his best outlook on a season since 2008, when he won his first Grand Slam title, also in Australia.

ENLARGE

A mock-up of Nadal (left) and Roger Federer as (gluten-free) steaks.
Photo Illustration by Patrick Conlon / Wall St. Journal

But no one would have predicted what has transpired since January. Djokovic's season has gone from good to great to outrageously, impossibly, unrealistically phenomenal. In an age when even small sports achievements can get enormous hype, there's really no superlative to describe what the soon-to-be 24-year-old has done this year.

"He has massively improved his serve, his fitness, and he's hitting his forehand a lot better," said Brad Gilbert, the ESPN commentator and former coach of Andy Murray and Andre Agassi. "I've never seen anybody move better on a tennis court in my life. He's so flexible, he can make impossible gets with ease."

Of Djokovic's 37 wins, 13 are against Top 10 players, including four against Nadal and three against Federer, who in all his years of dominance never started a season in so grand a fashion. If Djokovic reaches the French Open final, he could have 43 consecutive victories—one more than John McEnroe's record 42 to start 1984 (that streak ended in the French Open final, after McEnroe won the first two sets against Ivan Lendl).

Djokovic's 2011 on-court stats border on the absurd: He has won 89% of his service games, 43% of his return games and half of his break points. In his four matches against Nadal, he has repeatedly gotten the better of the Spaniard in rallies lasting longer than eight shots. No one has done that to Nadal in his professional career.

After Nadal lost to Djokovic in Indian Wells in March, Nadal spoke about the importance of confidence and how quickly it can fade. "We will see when [Djokovic] loses the first time," he said. "I don't know if [it's] gonna happen in Miami or happen in Monte Carlo or not gonna happen." At the time, the remark inspired laughter—surely Djokovic, like everyone else, would lose again. All you need to know about tennis in 2011 is that it now seems plausible that he won't.

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